The invention relates generally to steelmaking and, more particularly, to a method of preheating steelmaking ladles. As noted in U.S. Pat. No. 5,981,917, in steelmaking brick or cast refractory-lined ladles are used to hold the molten steel during steelmaking from an iron source, e.g., in an electric arc furnace, and to transport the molten steel to the next stage in steel processing, such as a continuous caster. These ladles may be large enough to hold 30 to 200 tons, or more, of molten steel. Since steelmaking is typically carried out continuously, several ladles are rotated through the melt shop and casting shop simultaneously. There are also generally ladles which are off line in reserve and for repair and maintenance.
The thermal state of the ladles has a direct and significant impact on the length of the campaign in making the steel. The refractories of the ladle must be heated to the same temperature, typically about 2700 to 2900° F., as the molten steel in it. The ladles even when direct recycled through the melt and casting shops will cool as the molten steel is discharged into the caster, and cool farther before the ladle is returned for recharging in the melt shop. Moreover, if ladles are taken off line in the steelmaking cycle, they typically cool to ambient temperatures, and the replacement ladles have to be heated from ambient temperature to operating temperature. In any case, ladles may be preheated to reduce the length of the campaign during steelmaking and increase the steelmaking capacity of the melt shop and the entire steelmaking facility.
In short, steelmaking ladles must be heated up when filled with molten metal because of the heat absorbed from the melt by the ladle refractory lining. On the other hand, the ladles cool down when empty. Moreover, the length of time during which a ladle is empty is highly variable and unpredictable. Delays due to a major ladle repair take many hours to complete and result in a cold ladle. If used in that condition, the steelmaking campaign will be considerably lengthened since the ladles must be heated with the molten metal to steelmaking temperature. Further, the temperature may be critical to the casting operation as molten steel may need to be introduced into the caster tundish at controlled temperature near liquidus metal temperature, say about 40° F. above the liquidus metal temperature. Thus, it is quite significant to operating capacity and the energy efficiency of the steelmaking plant that the heating and heat loss of ladles be closely controlled.
As a result, preheating of ladles before charging in the melt shop has become a common practice. Particularly, ladle preheating served to reduce damage to ladles taken out of the rotational cycle for repair and maintenance and for ladles first introduced into use. In any event, preheating reduced thermal stresses in the ladle refractory, and reduced the length of steelmaking campaigns and correspondingly increased the capacity of the steelmaking plant. However, overheating of preheated ladles also occurred which resulted in costly energy losses and resulted in unwanted and expensive refractory damage.
Usually preheating of ladles was performed with a gas-fired burner which injected a combustion flame into the interior of the ladle. Gas-fired ladle preheaters are represented, for example, by U.S. Pat. Nos. 4,359,209; 4,229,211; 4,014,532, and 3,907,260. Such a preheating apparatus may preheat the ladle to a desired temperature such as a temperature between 1800° F. and 2000° F. The current temperature of the ladle during the preheating process was often measured and controlled using a thermocouple (see, e.g., U.S. Pat. No. 4,718,643) or pyrometer (see, e.g., U.S. Pat. No. 4,462,698). As a result, conventional ladle preheating processes have involved consumption of large amounts of fuel, such as natural gas, and have resulted in damage to the refractories from overheating.
Accordingly, there is an unmet need for a method to reduce the amount of fuel consumed during preheating of the ladle refractories for use in steelmaking, and also to preheat the refractories of the ladle to a desired temperature efficiently while inhibiting damage and wear of the refractories from overheating.
Disclosed is a method of preheating a steelmaking ladle having an open upper portion and inner refractory surfaces. The method comprising the steps of:                (a) positioning a preheater having a radiant reflective surface and at least one burner adjacent the open upper portion of the steelmaking ladle where the reflective surface comprises an emissive coating;        (b) heating the inner refractory surfaces of the steelmaking ladle to a desired temperature by combustion through the burner of the preheater where the emissive coating of the reflective surface facilitates preheating of the steelmaking ladle;        (c) positioning a pyrometer to measure a representative temperature of the inner refractory surfaces of the steelmaking ladle during heating;        (d) generating electrical signals indicative of the representative temperature of the inner refractory surfaces of the steelmaking ladle measured by the pyrometer; and        (e) controlling the temperature of the heating by the preheater of the inner refractory surfaces of the steelmaking ladle using the electrical signals generated by the pyrometer.        
The method of preheating a steelmaking ladle may have the open upper portion of the steelmaking ladle positioned substantially opposite the reflective surface with the emissive coating of the preheater, and the reflective surface may substantially cover the open upper portion of the steelmaking ladle. Also, a gap of no more than 8 inches or 3 inches may be maintained between the reflective surface of the preheater and the open upper portion of the steelmaking ladle.
The emissive coating used in the method of ladle preheating may be disposed on a refractory surface of the preheater, and the refractory surface may substantially cover the open upper portion of the steelmaking ladle. The emissive coating may have an emissivity greater than 0.85 or 0.90, or may be between 0.85 and 0.95.
The emissive coating used in the method of ladle preheating may be a silicide coating. Further, the silicide coating may be selected from the group consisting of molybdenum silicide, tantalum silicide, niobium silicide or a combination thereof.
Disclosed is a method of preheating steelmaking ladles using a heating unit with a burner. The method comprising the additional step of regulating a flow rate of fuel to the burner during an idle state of the burner between preheating cycles, where the flow rate of the fuel is set to no higher than 600 SCFH during the idle state.
Disclosed is a method of preheating steelmaking ladles using a heating unit with a burner. The method comprising the additional step of regulating a flow rate of fuel to the burner during an idle state of the burner between preheating cycles, where the heating unit includes a direct drive throttle valve for regulating the flow rate of the fuel to the burner.
Numerous additional advantages and features will become readily apparent from the following detailed description of exemplary embodiments, from the claims and from the accompanying drawings.